The periodic table organizes all known chemical elements in a grid based on shared characteristics. Elements are placed into rows (periods) and columns (groups). Elements within a single vertical column, or group, share the same number of valence electrons—the electrons in the outermost shell of an atom. This shared electronic structure determines how an element interacts with others. This article examines the distinct nature and examples of elements found in Group 1A and Group 7A, two columns that represent extremes of chemical reactivity.
Group 1A: Defining the Alkali Metals
Group 1A of the periodic table, excluding hydrogen, is comprised of the alkali metals: lithium (\(\text{Li}\)), sodium (\(\text{Na}\)), potassium (\(\text{K}\)), rubidium (\(\text{Rb}\)), cesium (\(\text{Cs}\)), and francium (\(\text{Fr}\)). These elements are soft, silvery metals that can be easily cut with a knife. They possess low densities; the first three (lithium, sodium, and potassium) will float on water as they react. Chemically, they are highly reactive, with reactivity increasing down the group, often reacting violently with water to produce a strong base and hydrogen gas.
These metals are rarely found in their pure elemental form in nature, existing almost exclusively in compounds. Sodium is a component of table salt (sodium chloride) and plays a major role in nerve function and fluid balance in the human body. Potassium is found in fertilizers to promote plant growth and is a necessary nutrient for maintaining heart rhythm and muscle contraction. Lithium has become a key element in modern technology, particularly in the lightweight batteries used in electric vehicles and portable electronics.
Group 7A: Defining the Halogens
Located on the opposite side of the periodic table, Group 7A consists of the halogens: fluorine (\(\text{F}\)), chlorine (\(\text{Cl}\)), bromine (\(\text{Br}\)), iodine (\(\text{I}\)), and astatine (\(\text{At}\)). These non-metals are highly reactive and are rarely encountered in their elemental form in nature. Halogens exist as diatomic molecules, meaning two atoms bond together in their pure state (e.g., \(\text{F}_2\) or \(\text{Cl}_2\)). At room temperature, they exhibit a range of physical states: fluorine and chlorine are gases, bromine is a volatile liquid, and iodine is a solid.
Halogens are used widely for their chemical properties. Chlorine is used globally for water purification and disinfection, killing bacteria in drinking water and swimming pools. Fluorine is often added to municipal water supplies and toothpaste, where it helps strengthen tooth enamel and prevent decay. Iodine is used in medicine, often applied topically as a mild antiseptic to clean minor wounds and prevent infection.
The Basis of Their Extreme Reactivity
The reactivity of Group 1A and Group 7A elements is directly explained by their valence electron count. Atoms are most stable when their outermost electron shell is completely full, known as achieving a stable electron octet. Group 1A elements have only a single valence electron. It is energetically favorable for these metals to lose that electron to achieve the stable, full shell of the preceding noble gas.
Conversely, Group 7A elements have seven valence electrons, meaning they are just one electron short of a complete octet. They have a strong drive to gain a single electron to fill their outer shell and reach maximum stability. The Group 1A metals’ eagerness to donate an electron perfectly complements the Group 7A non-metals’ eagerness to accept one. This intense desire to lose or gain an electron, rather than share, is why both groups are so chemically active.
The Formation of Common Salts
The electronic imbalance between Group 1A and Group 7A elements leads to ionic bonding. When an alkali metal atom contacts a halogen atom, the metal readily transfers its single valence electron to the non-metal. This electron transfer results in the metal atom becoming a positively charged ion (cation) and the halogen atom becoming a negatively charged ion (anion). The resulting ions are stable because they have achieved a full outer electron shell, mimicking the configuration of a noble gas.
The strong electrostatic attraction between these positive and negative ions holds them together, creating a neutral compound called a salt. The most famous example is the reaction between sodium and chlorine, which forms sodium chloride (\(\text{NaCl}\)), commonly known as table salt. Other examples include the combination of potassium and chlorine to form potassium chloride (\(\text{KCl}\)), or the reaction between lithium and fluorine to yield lithium fluoride (\(\text{LiF}\)).